1. Field of the Invention
This invention relates to alignment systems for aligning a pattern on one object to a pattern on another object, and more particularly relates to alignment method and apparatus for aligning a mask with a wafer in a high-resolution microlithography system, having a high-sensitivity mask-wafer displacement detection system characterized by an optical heterodyne technique that uses two laser frequencies to generate by diffraction and interference a displacement signal at a different frequency, and having a high-precision laser-modulation mask-wafer positioning system characterized by controlling the cavity spacing of a highly stable optical resonator with a modulated frequency-stabilized laser, making it possible to align a wafer pattern with a mask pattern with a precision on the order of a few nanometers.
2. Description of the Prior Art
Lithography systems are extensively used in the production of integrated circuit chips and electronic circuit boards. Such systems typically include an exposure source such as a high intensity lamp or a laser or source of other radiation, mask and substrate positioning systems, a projection system to illuminate and image the pattern present on the mask onto the substrate, and an alignment system to accurately position the mask and the wafer with respect to each other prior to exposure. The intent typically is to illuminate a wafer coated with a layer of a radiation-sensitive material so as to produce the desired circuit pattern, which later will be metallized or otherwise activated during further processing. During production of integrated circuit chips, a wafer typically undergoes numerous physical treatment steps following illumination through several different masks. This requires that prior to exposure using each new mask, the wafer position be precisely realigned with respect to the new mask so as to carry out each process step at the desired microscopic physical locations as dictated by the preceding process steps. The precision of such alignment typically must be a small fraction of the minimum feature size of the circuit patterns.
As the demand for chips with ever greater memory and processing power increases, the individual bits on the chips get smaller in size. This requires that the lithography equipment used for imaging these patterns have higher and higher resolution. Simultaneously, the reduced minimum feature size of the circuit patterns demands that successive mask levels be aligned with respect to each other with ever greater accuracy. For example, in production of 1 Mb (megabit) and 4 Mb memory chips using, respectively, 1 micron and 0.7 micron geometries, the lithography system typically must have an alignment precision of 0.3 micron and 0.2 micron, respectively. Lithography systems for producing 16 Mb devices will need to have an alignment capability of 0.15 micron, and it is expected that production of 64 Mb chips will demand that lithography apparatus possess alignment precision of 0.1 micron or better. Performance of current alignment technology as seen in today's lithography tools is limited in accuracy in the 0.2-0.3 micron range.
Prior art alignment approaches have used various techniques for sensing the relative positions of the mask and the wafer. All approaches rely on detecting certain targets, fabricated on the mask and the wafer prior to the alignment step. The targets are illuminated by light and through scattering, reflection, diffraction or interference, they produce light signals which are detected and analyzed to determine the relative displacement required between the mask and the wafer to bring them in the desired alignment. A common approach found in the prior art uses mask and wafer targets that are sets of etched lines in a reflective background and arranged in the form of squares or chevrons. The targets are illuminated with visible light and the beams reflected from them are observed simultaneously in a viewing system. In the viewing microscopes the target backgrounds appear bright due to directly reflected light whereas the etched lines appear dark due to light being scattered out of the collection angles of the microscope objectives. The alignment process consists in aligning the line patterns of the two targets as seen in the viewing system by producing the appropriate relative displacement between the mask and the wafer using a conventional laser interferometer. This alignment approach achieves a precision of 0.25-0.30 micron.
Another approach used in prior art, known as dark-field alignment method, consists in fabricating on the wafer a pattern that is a set of rectangles and fabricating on the mask a pattern that is a set of parallel slits which, under perfect mask-wafer alignment, are imaged on the edges of the rectangles on the wafer. The lens which collects the light scattered from the wafer is so apertured that all specular reflection, i.e. directly reflected light, is blocked. Thus, the detector sees only those light rays which are scattered from the edges of the rectangular targets on the wafer. By optimizing the mask and wafer positions such that the transmission of these scattered rays through the slits on the mask is maximized, the mask and the wafer are brought in alignment. The required relative displacement between the mask and the wafer is accomplished, as above, by use of a conventional laser interferometer, delivering an overall alignment accuracy in the 0.20-0.25 micron range.
Other prior art approaches have used diffraction gratings as alignment targets. The intensity or phase information in light diffracted from a wafer grating (and in some cases, again through a mask grating) is appropriately analyzed so as determine the relative mask-wafer displacement necessary to produce alignment. Again, in systems using diffraction grating targets, the relative mask-wafer displacement is carried out using conventional laser interferometers, and the alignment precision achieved lies in the vicinity of 0.20 micron. Although optical phenomena and techniques exist that in principle are sensitive to finer physical displacements, none have been developed in a way that makes them suitable for lithography alignment systems.
In view of the limitations in prior art as discussed above, there is an important need to develop a superior alignment system for high-resolution lithography machines to deliver greater mask-wafer alignment precision that will be required in the fabrication of higher-density microelectronic devices.